1. Field of the Invention
This invention relates to a steering apparatus for a vehicle. More particularly, it relates to a vehicular steering apparatus wherein wheels are turned in such a way that a sub steering angle which is electrically controllable by a sub steering mechanism is mechanically added to the steering angle of a steering wheel manipulated by a driver.
2. Description of the Related Art
Heretofore, there have been known a large number of steering apparatuses in each of which a sub-steering-angle superposition mechanism is mounted between a steering wheel and steered wheels, and the transmission characteristic of the turning angle of the steered wheels versus the steering angle of the steering wheel manipulated by a driver is changed in accordance with the traveling state of a vehicle. (Refer to, for example, JP-A-2005-41363 which shall be termed “Patent Document 1” below.)
In the steering apparatus of Patent Document 1, it is disclosed that a PWM duty is calculated from the deviation between the target revolution angle of an electric motor for driving the sub-steering-angle superposition mechanism and a detected revolution angle, whereupon the electric motor is driven and controlled on the basis of the PWM duty.
Besides, in JP-A-11-208499 (termed “Patent Document 2” below), it is disclosed that the followup delay of the sub-steering-angle superposition mechanism is detected from the deviation between the target steered angle of the steered wheels, which has been calculated from the steering angle of the steering wheel and the transmission characteristic, and a detected steered angle, whereupon the transmission characteristic or the target steered angle is corrected so as to decrease the deviation.
In the steering apparatus of Patent Document 1, the PWM duty is calculated from the deviation between the target revolution angle of the electric motor for driving the sub-steering-angle superposition mechanism and the detected revolution angle, and the revolution angle of the electric motor is feedback-controlled. However, the PWM duty and a current which flows through the electric motor are not uniquely determined on account of an induced voltage ascribable to the revolution of the electric motor, etc. It is therefore difficult to generate by the electric motor, a torque which agrees with a necessary torque obtained from the deviation of the revolution angles. That is, there has been the problem that the setting of control constants for use in the calculation for obtaining the PWM duty from the deviation of the revolution angles is difficult.
Besides, in Patent Document 2, the followup delay of the sub-steering-angle superposition mechanism is detected from the deviation between the target steered angle and the detected steered angle, and hence, there has been the problem that the followup delay has already been involved at the stage of the detection.